Differences in Process Evaluation Coverage

 The coverage of welding procedure qualification is closely related to the basic variables of welding, which are the main parameters of the welding method. If the change exceeds a certain limit, a new welding procedure must be established. Some basic variables are determined in the systems of NB/T 47014-2011 and GB/T 22652-2019, and these basic variables include welding method, the thickness of surfacing layers, types and thickness of base metal, filler metal, welding positions, heat treatment and welding process measures. They must be written in the welding procedure specification (WPS), and the scope of each variable should be indicated. The test conditions are recorded in another document, that is, the Process Qualification Report (PQR). A PQR may correspond to a series of WPS documents, or vice versa. However, GB/T 22652-2019 stipulates that the welding rod, alloy powder or welding wire shall comply with GB/T 984-2001, JB/T 4744-2006 and NB/T 47044-2014, which is different from NB/T 47014-2011.
 

(1) Welding methods

Surfacing of sealing surfaces of valves include plasma arc surfacing, oxy-fuel gas surfacing, electrode arc surfacing, tungsten gas shielded surfacing, MIG surfacing and submerged arc surfacing. NB/ T 47014-2011 includes electroslag surfacing. When the welding method is changed, the process qualification should be re-qualified.
 

(2) Filler metals

In the evaluation standard of surfacing welding procedure, NB/T 47014-2011 and GB/T 22652-2019 list 12 factors such as changing the flux type, powder feeding rate and volume change of filler metal. However, the welding materials are not classified in NB/T 47014-2011. When the standard model of the electrode changes, it needs to be re-evaluated. GB/T 22652-2019 classifies the welding materials, and the category number of the electrode changed should be re-evaluated.
 

(3) Base materials

The purpose of classifying and grouping base materials is to reduce the number of welding procedure evaluations. For this reason, steels with similar chemical composition, mechanical properties and welding properties are grouped into the same category, while ferrous metal base metals with the requirement of impact toughness are specified under the category number. This grouping is based on an analogy with base metal properties such as composition, weldability, brazability and mechanical properties. This grouping does not mean that a base metal used in the evaluation test can be replaced by other base metals without distinction, regardless of metallurgical properties, post-weld heat treatment, design, mechanical properties and operating requirements to consider its suitability. NB/T 47014-2011 classifies and groups the materials, and stipulates that changing the category number of the base material requires re-assessment of welding procedures, and GB/T 22652-2019 also has the same requirement. The thickness of the base material is an important factor, and the coverage of the base material thickness in NB/T 47014-2011 and GB/T 22652-2019 is the same.
 

(4) Welding seam specimens

Weld seam specimens can be divided into two types, that is, the plate type and the tubular type. Both types are acceptable in NB/T 47014-2011 and GB/T 22652-2019. The preparation requirements of the two standards are the same, but the expression is slightly different. NB/T 47014-2011 is more detailed.
 

(5) Welding positions

The welding position is also a welding process evaluation factor. Although the current of vertical welding is reduced, the welding speed is also greatly reduced, and the welding heat input is greatly increased. In NB/T 47014-2011 and GB/T 22652-2019, the provisions on the qualification of surfacing welding procedures are consistent, and the standards stipulate that in addition to the qualification of horizontal welding, vertical welding or overhead welding positions, they can also be used for flat welding positions. The qualified welding position can also be changed. NB/T 47014-2011 describes the qualification of 5G and 6G tubular specimens. The 5G qualified surfacing welding process is suitable for flat welding, vertical welding and overhead welding, and the 6G qualified surfacing welding process is suitable for all welding positions.
 

(6) Preheating and post-weld heat treatment

The thermal process that can change the structure, properties or residual stress of the welded joint after welding is called post-weld heat treatment. NB/T 47014-2011 stipulates that the preheating temperature is lower than the rated value by more than 50°C, and when the maximum inter-pass temperature is higher than the rated record value, the welding process needs to be re-evaluated; when the time increases by more than 25% of the rated value, the welding procedure needs to be re-qualified or the category of the post-welding is changed.
 

(7) Shielding gas

NB/T 47014-2011 and GB/T22652-2019 stipulate that when changing the type and flow of shielding gas, changing the proportion of mixed shielding gas, and canceling the protective gases need to be re-evaluated for plasma arc surfacing welding and gas shielded welding surfacing welding.
 

(8) Welding electrical characteristics and welding process measures

The electrical characteristics of welding in the two standards are the same, mainly including changing the type or polarity of the current. When surfacing the first layer, the heat input or the volume of the deposited metal per unit length of the weld bead exceeds 10% of the rated value. For plasma arc welding, if there are changes in current changes by 10%, voltage changes by 10%, power changes by 10%, and changes in factors such as electrode arc welding current increases by 10%, it needs to be evaluated again. The welding process measures in the two standards are the same, mainly including the change of multi-layer surfacing welding to single-layer surfacing welding, canceling the magnetic field control of the welding pool, changing the number of electrodes in the same molten pool, increasing or canceling the electrode swing, which all need to be re-evaluated.